t0116: implement diretag checks

[laurentsenta]

Closes #93

Rationale:

The API in #96 relies on having implicit coupling (side effects and time) but the rest of the API is declarative.

This is a demo of how we could do the same without changing the API.

I would favor that approach:

• we don't endorse side-effects and transforming requests at the API level, we'd have a single "rogue" test,
• the side-effects are simple and explicit (we call Run twice, and we create requests in the regular way)
• no API change

[galargh]

👏